This invention relates to a process for deinking of recycled paper to produce new, usable paper. The invention further relates to such a process using a pressurized deinking module. More particularly, the invention relates to the use of particular imidazolinium surfactants to obtain a more effective ink removal and a brighter paper stock.
Recycling of paper products such as newspapers, magazines, books, handbills and the like, has long been practiced for reasons of economy. In comparison to starting from basic cellulosic materials such as wood or rags, there are fewer steps in the recycling process, and the raw materials are less expensive. In the earlier days of recycling, it was mainly a hit or miss matter, making use of whatever materials may have been available. In more recent times, other factors have made recycling of paper more prevalent. There has been a veritable explosion of publishing of printed materials, particularly magazines and paperback books (in spite of increased television viewing). Office copiers turn out a considerable amount of waste printed matter. All of this material has created a serious disposal problem with overflowing trash dumps and consequent effects on the environment. The environmental problems have resulted in mandatory recycling legislation in many jurisdictions. Thus there is now available a much greater amount of waste paper for recycling, which is being collected outside the paper industry and is therefore more readily available to the industry for recycling. Thus the economic reasons for recycling are now even more compelling, reinforced by the environmental factors. Thus any improvement in processes for recycling waste paper are highly desirable.
Earlier processes for recycling printed waste paper included certain basic steps, namely cutting the waste paper and dispersing it in an aqueous medium containing flotation surfactants to form a paper pulp slurry, then submitting the pulp slurry to aeration and flotation to separate the ink. The aeration and flotation are carried out in the same vessel, open to the atmosphere. The pulp slurry, together with air, is fed into the vessel where the air forms bubbles to which ink particles become attached. The bubbles and ink rise to the surface as a foam. The flotation surfactants in the pulp slurry facilitate adherence of the ink particles to the air bubbles. The vessel has an overflow area on one side where the foam and some of the pulp slurry flow out of the vessel as reject material. The air originally pumped into the pulp slurry passes into the atmosphere. The major portion of the pulp slurry is withdrawn from the bottom of the vessel as acceptable recycled pulp. This pulp could then be submitted to further treatments including further contaminant and ink removal and bleaching. This system presents some problems in that the overflow of some of the pulp slurry results in loss of useful paper fiber. Moreover, the escaping air is an environmental problem since it contaminates the atmosphere. In addition, the paper made from the pulp of this process was not sufficiently bright for use in many commercial applications. In view of the importance of waste paper recycling, numerous improvements have been proposed, such as new chemicals to enhance the flotation of the ink in the deinking step of the process. However, the basic process was not changed, and the inherent problems of the open system, viz. loss of pulp, atmospheric contamination, and poor brightness, remained until quite recently.
Around 1987, a new concept in the flotation step of the waste paper recycling process was developed by the Beloit Corporation. This concept is described in detail in an article entitled "Pressurized Deinking Module", Pulp Technology and Energy, May, 1990. The new concept is to conduct the flotation step under pressure in a closed, pressurized vessel. The vessel, known as a pressurized deinking module (PDM), is used to perform the deinking flotation procedure under pressure. Briefly, the Beloit PDM consists of a roughly cylindrical, elongated vessel, closed at both ends, horizontally inclined with an inlet conduit smaller than the main cylindrical vessel at one end, and outlets at the other end for removing pulp accepts and foam rejects. A preformed pulp slurry containing flotation surfactants is pumped into the inlet conduit which forms an aeration and mixing zone, followed by the flotation zone in the main cylindrical vessel. Air is pumped into the aeration zone under pressure, and mixing of the air and pulp slurry take place. The mixing is vigorous, resulting in smaller than usual, and therefore a greater number of, air bubbles than in a conventional non-pressurized deinking system. In the flotation zone, the greater number of air bubbles increases the chance of contact with ink particles. The bubbles then rise to the surface, forming a foam of air and ink particles in a layer on top of the pulp slurry. The larger number of bubbles removes a larger amount of the ink particles, resulting in a whiter and brighter paper stock. The pulp slurry is removed from a point below the surface of the pulp slurry at the outlet end of the pressurized deinking module. The air and ink particle foam rejects are forced out of an outlet which is above the level of the pulp slurry and within the foam layer formed by the air under pressure within the vessel. The foam rejects are then fed to a cyclone separator to separate the ink particles from the air. The air can then be released to the atmosphere since it is not contaminated. Alternatively, it may be recycled to the aeration zone of the pressurized deinking module. Since the vessel is totally enclosed, all factors, such as air flow rate in and out, pulp slurry in and out, and air bubble and ink particle contact, are controllable, making for a much more effective flotation process. In addition, since no pulp slurry leaves the vessel with the ink foam, as in the open flotation process, the yield of recycled paper stock is higher. Thus, the pressurized deinking module solves the long standing problems of releasing contaminated air, and loss of pulp slurry by overflow, as well as producing a brighter paper stock.
The ideal objective of a deinking process is to obtain complete ink removal with no fiber loss. This situation has to date never been achieved by any of the above described systems.
It is therefore an object of this invention to provide a process for recycling waste paper having an improved deinking step to produce a recycled paper which has a higher degree of brightness than that previously produced.
A further object is to provide such a process which results in a higher yield of recycled fiber.
Another object is to provide such a process requiring fewer steps and reduced chemical additives.